Optical fiber including a fluorescent dopant

ABSTRACT

The present invention relates to an optical fiber comprising an optical core (6) based on silica for guiding the majority of lightwaves and containing at least one fluorescent dopant, surrounded by optical cladding (5) likewise based on silica, and having a refractive index lower than that of said core, said core comprising: a central zone (2) of radius a 0 , containing said fluorescent dopant; and a peripheral zone (4) of radius a, surrounding said central zone, having a refractive index greater than that of said cladding, and not containing fluorescent dopant; the fiber being characterized in that said core (6) further comprises an intermediate zone (3) of radius a 1  and having a softening temperature greater than that of central zone (2), said intermediate zone (3) being such that a 0  &lt;a 1  &lt;a and not containing any fluorescent dopant.

The present invention relates to an optical fiber including at least onefluorescent dopant, in particular a fiber adapted to be used in opticalfiber amplifiers or lasers.

Apparatuses in which a fiber is used containing a fluorescent dopantcapable of interacting with an excitation signal, referred to as a"pumping" signal for the purpose of obtaining a desired output signal,i.e. an optical oscillator or an optical amplifier, are subject to agreat deal of study at present. The same type of optical fiber is usedboth in lasers and in amplifiers: the fiber is an optical fiber ofconventional structure having an optical core for guiding the majorityof the lightwaves, doped by means of a fluorescent dopant, andsurrounded by optical cladding. Two types of signal are conveyed by suchan optical fiber: pumping signals and useful signals conveyinginformation, referred to as "signals to be transmitted". The fluorescentdopant may be an element having three or four transition levels. By wayof example it may be a rare earth such as erbium.

The operation of apparatus making use of the fluorescence effect relieson the following basic principle: ions of fluorescent dopant, referredto as "active" ions, initially in their fundamental state, absorb thephotons of the pumping signal, thereby transferring them to an excitedhigher transition level; this phenomenon is known as "populationinversion", and the dopant ions in this excited state are said to be"inverted". From this excited level, inverted ions can subsequentlyreturn to their fundamental state by emitting a photon, i.e. by means ofa laser transition. In an optical amplifier or laser type apparatus,such emission is stimulated by the presence of a photon of a signal tobe transmitted, and consequently the operation of such apparatus alsodepends on the interaction between active ions and photons in saidsignal at the wavelength thereof.

In order to obtain the population inversion that is essential for laseroperation, it is necessary to pump at least half the active ions fromtheir fundamental state to the excited higher level. At any particularpoint of the core of the optical fiber, if less than half of the activeions are inverted, then the signal to be transmitted is attenuated atsaid point because the non-inverted active ions absorb photons.

Consequently, in order to make best use of the pumping power injectedinto an optical fiber doped by means of a fluorescent dopant, it ispreferable to confine the active dopant within the zone of the fiberthat coincides with the peak intensity of pumping, i.e. around the axisof the fiber, and to avoid having any active ions located where thepumping intensity is weaker.

The intensity maximum of a monomode signal lies likewise on the axis ofthe fiber, so a monomode signal interacts effectively with the invertedions.

A known solution for confining the fluorescent dopant to where thepumping intensity is at a peak is described in U.S. Pat. No. 4,923,279.That solution consists in subdividing the core into two zones, an activecentral zone containing the fluorescent dopant, and a peripheral zone incontact with the central zone but not containing fluorescent dopant.

In conventional manner, the central zone is also doped with aluminum,firstly to avoid the effect whereby erbium ions can become segregated inthe core ("clustering"), where clustering considerably decrease theperformance of the doped fiber for reasons that need not be explained indetail herein, and secondly as an index-raising dopant. Also, in orderto obtain monomode guidance in the core, the peripheral zone is dopedwith one or more index-raising dopants. It is recalled that such dopantsraise the refractive index relative to that of the optical cladding,which generally has an index that is substantially equal to that of puresilica.

Nevertheless, that solution is not satisfactory.

Known index-raising dopants, and in particular aluminum, germanium, andphosphorus, also cause the softening temperature of the silica in whichthey are incorporated to decrease. Thus, the two adjacent zones of thecore (the central zone and the peripheral zone) have reduced softeningtemperatures.

Unfortunately, when the preform is manufactured by a chemical vapordeposition (CVD) method, the operation of collapsing the preform whichis performed at high temperature, higher than the softening temperatureof the various zones of the core, gives rise to significant softening ofthe core. As a result, the dopants in the various zones thereof tend tointerdiffuse, and in particular the fluorescent dopant migrates towardsthe peripheral zone of the core and is no longer confined in its centralzone. This has the highly unfortunately effect of reducing pumpingefficiency.

An object of the present invention is to avoid migration of thefluorescent dopant away from the active zone.

To this end, the invention provides an optical fiber comprising anoptical core based on silica for guiding the majority of lightwaves andcontaining at least one fluorescent dopant, surrounded by opticalcladding likewise based on silica, and having a refractive index lowerthan that of said core, said core comprising:

a central zone of radius a₀, containing said fluorescent dopant; and

a peripheral zone of radius a, surrounding said central zone, having arefractive index greater than that of said cladding, and not containingfluorescent dopant;

the fiber being characterized in that said core further comprises anintermediate zone of radius a₁ and having a softening temperaturegreater than that of central zone, said intermediate zone being suchthat a₀ <a₁ <a and not containing any fluorescent dopant.

The intermediate zone interposed between the central zone and theperipheral zone and having a softening temperature higher than that ofthe zone containing the fluorescent dopant thus constitutes a barrier tomigration of the dopant during the various operations in which thepreform is heated to above the softening temperature of the zonecontaining the index-raising dopants (and in particular during preformcollapsing). The fluorescent dopant thus remains confined to the centralzone of the core, so that the desired efficiency is indeed obtained.

In the prior art described and for a Gaussian distribution of pumpingintensity or of intensity of the signal to be transmitted, theconfinement factor ε of the fluorescent dopant is substantially equal tothe ratio of the radius of the active zone over the mode radius of thepumping signal or of the signal to be transmitted by the fiber, giving avalue of the order of 50%. With the fiber of the invention, theconfinement factor is less than 10%, which represents an improvement bya factor of five or more over the prior art.

Also, because of the invention, it is easy to control both the diameterof the active zone since the presence of the intermediate zone makes itpossible to avoid changing the initial confinement during the collapsingoperation, and the mode diameter which is a function, in particular, ofthe index difference between the peripheral zone and the cladding, andalso the core radius since there is no difficulty within conventionallyknown limits in adding index-raising dopants in said peripheral zonewithout that being harmful to confinement of the active dopant, as it isin the prior art.

Advantageously, to obtain a softening temperature of the intermediatezone higher than the softening temperature of the central zone of thecore, said intermediate zone contains little or no index-raising dopant(it is the peripheral zone that makes it possible to obtain the indexdifference that is required for signal confinement within the core ofthe optical fiber).

The softening temperature of the intermediate zone is thus controlled byan appropriate selection of dopants and of dopant concentrations.

It is also possible to select the softening temperature of theintermediate zone so that it is greater than that of the peripheralzone, thereby making it possible to avoid dopants diffusing from theperipheral zone towards the central portion of the core.

The invention is applicable generally and regardless of which three orfour transition level fluorescent dopant is used; it is particularlyadvantageous when the dopant, e.g. erbium, possesses three transitionlevels.

Other characteristics and advantages of the present invention appearfrom the following description of a fiber of the invention, given by wayof non-limiting illustration.

In the following figures:

FIG. 1 is a cross-section of an optical fiber of the invention;

FIG. 2 shows the index profile of the optical fiber of FIG. 1 in a firstvariant of the invention;

FIG. 3 shows the index profile of the optical fiber of FIG. 1 in asecond variant of the invention; and

FIG. 4 shows the index profile of the optical fiber of FIG. 1 in a thirdvariant of the invention.

In the figures, elements in common are given the same referencenumerals.

FIG. 1 is a cross-section through an optical fiber 1 of the inventionwhich comprises, disposed coaxially from the inside towards the outside:

a central zone 2 constituting the active zone, based on silicacontaining a fluorescent dopant such as erbium, together with aluminum,and possibly other index-raising dopants such as germanium, for example;

an intermediate zone 3 of the invention based on silica that is dopedlittle or not at all, such that its refractive index is less than thatof the central zone 2, and its softening temperature is greater thanthat of the central zone 2;

a peripheral zone 4 based on silica that is doped so that firstly itsrefractive index is greater than that of the intermediate zone 3 and notless than that of the central zone 2, and secondly its softeningtemperature is less than that of the central zone 2, e.g. using dopantssuch as germanium, phosphorus, etc. . . . ; and

optical cladding 5 based on silica that is not doped or that is doped sothat its refractive index is less than that of silica, e.g. by usingfluorine.

The core 6 of the optical fiber 1 is constituted by the central zone 2,the intermediate zone 3, and the peripheral zone 4.

The radius of the central zone 2 is written a₀, the radius of theintermediate zone 3 is written a₁, and the radius of the core 6 iswritten a. The difference of refractive index between the peripheralzone 4 and the cladding 5 is written Δn, and the difference between therefractive index of the intermediate zone 3 and that of the cladding 5is written Δn'.

FIG. 2 shows a first possible ideal index profile for the fiber 1 ofFIG. 1: the curve 20 shows the refractive index n as a function ofradius r, while the curve 21 shows the intensity I of the pumping signalor of the signal to be transmitted by the fiber 1 as a function ofradius r. In this variant, the intermediate zone 3 of the invention hasa refractive index substantially equal to that of the cladding 5 (i.e.Δn' is zero), and the peripheral zone 4 has a refractive index that isslightly less than that of the central zone 2. The refractive index ofthe cladding 5 is substantially equal to the refractive index of puresilica, n₀.

FIG. 3 shows a second possible ideal index profile for the fiber 1 ofFIG. 1: the curve 30 shows the refractive index n as a function ofradius r, while the curve 31 shows the intensity I of the pumping signalor of the signal to be transmitted by the fiber 1 as a function ofradius r. In this variant, the intermediate zone 3 of the invention hasa refractive index greater than that of the cladding 5, which issubstantially equal to that of pure silica (i.e. Δn' is not zero), butit is still well below that of the central zone 1 so that, in accordancewith the invention, it limits diffusion of the active dopant from thecentral zone 2 towards the intermediate zone 3 because of the differencein softening temperatures between the two zones. The peripheral zone 4has a refractive index that is slightly less than that of the centralzone 2.

Finally, FIG. 4 shows a third possible ideal index profile for the fiber1 of FIG. 1: the curve 40 shows the refractive index n as a function ofradius r, and the curve 41 shows the intensity I of the pumping signalor of the signal to be transmitted by the fiber 1 as a function ofradius r. In this variant, the central zone 2 and the peripheral zone 4have refractive indices that are substantially equal, and the cladding 5has a refractive index n_(g) lower than the index no of pure silica. Inthis profile, the index step constraints on the peripheral zone 4 areless than those of the profiles of FIGS. 2 and 3 because the indexdifference Δn is obtained in part because of the negative index of thecladding 5, i.e. it is no longer necessary to use such a highconcentration of index-raising dopants in the peripheral zone 4 as it isin the preceding variants.

From these three profiles, it can be seen that in an optical fiber ofthe invention it is possible for the central active zone 2 to be dopedin such a manner that its index is greater than that of the cladding 5,e.g. by using germanium in addition to the aluminum that must be presentin order to avoid erbium clustering. Even if that reduces the softeningtemperature of the central zone 2, because the intermediate zone 3retains a softening temperature that is considerably higher, it cancontinue to act as a barrier against diffusion of the fluorescentdopant.

It will be clear from the above that the invention makes it possible toreduce the confinement factor ε in two ways:

firstly by making it possible to conserve confinement of the fluorescentdopant while the preform is being heated, because of the intermediatezone 3 which prevents the fluorescent dopant from diffusing; and

secondly by allowing the mode diameter of the fiber 1 to be increased bymodifying the refractive index of the peripheral zone 4, i.e. bymodifying the concentrations of index-raising dopants so as to obtainthe desired index difference Δn.

It is possible to increase this mode diameter when the pumping intensityor the intensity of the signal to be transmitted has a relatively flatcentral peak, since under such circumstances the desired interaction isconserved firstly between the dopant ions and the pumping photons in thecentral zone, and simultaneously secondly between the inverted ions andthe photons of the signal to be transmitted throughout the core.

Also, it is recalled that in an amplifying optical fiber, it is notpossible to reduce the confinement factor without limit. The gain of theamplifier is proportional to the confinement factor, to theconcentration of inverted doping ions, and to the length of theamplifying fiber. Thus, as the confinement factor is decreased, otherthings remaining equal, gain decreases, and that should be avoided. Itis therefore necessary to compensate the decrease in confinement factorby increasing the length of the amplifying fiber since the concentrationof inverted dopant ions has an upper limit that cannot be exceeded forreasons that are not described in detail herein. Unfortunately, thelength of the amplifying fiber cannot be increased indefinitely eithersince the longer the fiber the greater the attenuation losses on thesignal to be transmitted.

It is therefore necessary to find a compromise between decreasing theconfinement factor, thereby making it possible to increase pumpingefficiency, and increasing amplifier gain. For this purpose, theconfinement factor can be considered as the ratio between the radius ofthe active zone and the mode radius for a Gaussian distribution ofpumping signal intensity or of intensity of the signal to betransmitted. When distribution is not Gaussian, a more general formulais used for the confinement factor, given by: ##EQU1## where φrepresents the guided electromagnetic field envelope.

The above-mentioned comprise leads to selecting a confinement factorthat is less than 50%. For such a confinement factor, the parameters a₀,a₁, a, Δn, and Δn' for an optical fiber of the invention can be selectedto lie in the following ranges:

1 μm≦a₀ ≦2 μm

2 μm≦a₁ ≦4 μm

2 μm≦a≦4 μm

20×10⁻³ ≦Δn<30×10⁻³

0≦Δn'≦20×10⁻³

Δn'<Δn

It will be observed that the person skilled in the art can performdigital simulations enabling the various parameters given above to beselected so as to obtain the desired confinement, mode diameter, andfield envelope.

Naturally, the present invention is not limited to the abovedescription, and any means may be replaced by technically equivalentmeans without going beyond the ambit of the invention.

We claim:
 1. An optical fiber comprising an optical core (6) based onsilica for guiding the majority of lightwaves and containing at leastone fluorescent dopant, surrounded by optical cladding (5) likewisebased on silica, and having a refractive index lower than that of saidcore, said core comprising:a central zone (2) of radius a₀, containingsaid fluorescent dopant; and a peripheral zone (4) of radius a,surrounding said central zone, having a refractive index greater thanthat of said cladding, and not containing fluorescent dopant; the fiberbeing characterized in that said core (6) further comprises anintermediate zone (3) of radius a₁ and having a softening temperaturegreater than that of central zone (2), said intermediate zone (3) beingsuch that a₀ <a₁ <a and not containing any fluorescent dopant.
 2. Anoptical fiber according to claim 1, characterized in that saidintermediate zone (3) has little or no doping with refractive indexincreasing dopants, such that its refractive index is greater than orequal to that of said optical cladding (5), and less than that of saidperipheral zone (4).
 3. An optical fiber according to claim 1,characterized in that said peripheral zone (4) has a refractive indexless than or substantially equal to that of said central zone (2).
 4. Anoptical fiber according to claim 1, characterized in that said opticalcladding (5) has a refractive index less than or substantially equal tothat of pure silica.
 5. An optical fiber according to claim 1,characterized in that said intermediate zone (3) has a softeningtemperature greater than that of said peripheral zone (4).
 6. An opticalfiber according to claim 1, characterized in that said intermediate zone(3) has a refractive index equal to or slightly greater than that ofsaid cladding (5).
 7. An optical fiber according to claim 1,characterized in that it is monomode at the wavelength of the signal itis to transmit, and also at the pump wavelength of said fluorescentdopant.
 8. An optical fiber according to claim 1, characterized in thatsaid fluorescent dopant has three transition levels.
 9. An optical fiberaccording to 1, characterized in that said dopant is a rare earth. 10.An optical fiber according to claim 9, characterized in that said dopantis erbium.
 11. An optical fiber according to claim 1, characterized inthat a₀ lies in the range 1 μm to 2 μm, a lies in the range 2 μm to 4μm, and a₁ lies in the range 2 μm to 4 μm, and in that the indexdifference Δn between the refractive index of said intermediate zone (3)and that of said cladding (5) lies in the range 20×10⁻³ and 30×10⁻³. 12.An optical fiber according to claim 11, characterized in that the indexdifference Δn' between the refractive index of said peripheral zone (4)and that of said cladding (5) lies in the range 0 and 20×10⁻³.